A method and apparatus for measuring the biological condition of an animal by acquiring and analyzing its biological signals are provided. The biological signals from skin temperature, a photoplenthysmogram (PPG), an electrocardiogram (ECG), electrodermal activity (EDA), an electromyogram (EMG), and an electrogastrogram (EGG) are detected using a biological signal detection unit which is attached to the animal's skin. Feature vectors, including the mean heart rate of the photoplenthysmogram and its standard deviation, the very low frequency, low frequency, and high frequency components of heart rate variability, the frequency and mean amplitude of skin conductance responses, and the mean and maximum skin temperatures, are extracted from the detected biological signals. The biological condition, including needs and emotions, of the animal as to whether or not the animal feels hunger or fear, how much the animal is stressed, or whether or not the animal needs to have a bowel movement, is determined using a pattern classifier which has learned reference vectors, which reflect the behaviors, needs, and emotions of different kinds of animals for various biological conditions and are stored in a predetermined database. Therefore, the biological condition of the animal can be determined through instrumental communication, not through human languages, and the breeding of pets can be efficiently managed.